Cardiac defects are the most common congenital anomalies affecting nearly 1% of all live births. Despite major advances in the treatment of congenital heart disease (CHD), it remains the leading cause of death due to congenital anomalies in the newborn period. CHD results from a myriad of structural anomalies that present over a broad spectrum. Single ventricle anomalies make up one of the largest groups of cardiac anomalies resulting in severe, life-threatening disease. Single ventricle anomalies are made up of a group of cardiac defects that are dramatically different from each other structurally, but share a common feature that only one of two ventricles is of adequate functional size. Some of the cardiac anomalies that result in single ventricle physiology include: tricuspid atresia, pulmonary atresia, and hypoplastic left heart syndrome. This group of congenital cardiovascular anomalies results in mixing of deoxygenated blood from the pulmonary circulation with oxygenated blood from the systemic circulation. The circulation of deoxygenated blood through the systemic circulation causes chronic hypoxia and cyanosis, a bluish discoloration of the skin resulting in the moniker “blue baby syndrome”. Mixing of blood between the pulmonary and systemic circulation can also cause volume overload to the ventricle that if untreated can lead to heart failure. Untreated single ventricle cardiac anomalies are associated with 70% mortality during the first year of life (Samanek, Pediatr. Cardiol., 13:152-8 (1992)). The treatment of choice for single ventricle anomaly is surgical reconstruction (Giannico, et al., J. Amer. College Card., 47(10):2065-73 (2006); Petrossian, et al., J. Thorac. Cardiovasc. Surg., 132:1054-63 (2006)). Without surgery, survival into adulthood is unusual (Hager, et al., J. Thorac. Cardiovasc. Surg., 123:1214-7 (2002)).
Despite the dramatic structural differences in the cardiac defects causing single ventricle physiology, the ultimate plans for staged surgical reconstruction are actually quite similar. The goal of this series of staged operations is to separate the pulmonary circulation from the systemic circulation. This eliminates the mixing of systemic and pulmonary blood flow, resulting in improved systemic oxygenation, and preventing volume overload, thus normalizing the volume work of the systemic ventricle, thereby preventing heart failure. This is accomplished through a series of staged operations designed to reconstruct the cardiovascular structures so that the single ventricle pumps oxygenated blood through the systemic circulation. The deoxygenated blood is then passively circulated through the pulmonary circulation where it is oxygenated and returned to the heart. This type of surgical procedure is referred to as a Fontan operation. The Fontan operation has undergone several modifications since it was first reported in 1971 (Fontan, et al., Thorax, 26(3):240-8 (1971)). The most commonly performed modification of the Fontan operation is the extra cardiac total cavopulmonary connection (EC TCPC). The modified Fontan operation is considered the standard of care for the treatment of patients with single ventricle cardiac anomalies and has substantially improved both the quality and long-term survival of these patients. However; it is still considered a palliative (non-curative) procedure with significant morbidity and mortality (Giannico, et al., J. Amer. College Card., 47(10):2065-73 (2006); Petrossian, et al., J. Thorac. Cardiovasc. Surg., 132:1054-63 (2006)). One important cause of morbidity and mortality in patients requiring the Fontan operation is the conduit used to connect the inferior vena cava to the right pulmonary artery when native tissue cannot be used (Jonas, et al., J. Thorac. Cardiovasc. Surg., 117:688-96 (1999)). When Fontan and Kirklin reviewed the late outcome of an early cohort of patients surviving the Fontan procedure, they concluded that much of the late morbidity could be attributed to problems associated with conduit use (Fontan, et al., Circulation, 81:1520-36 (1990)). It is widely accepted that the ideal conduit has not yet been developed (Conte, FASEB, 12:43-5 (1998); Kakisis, J. Vasa Surg., 41:349-54 (2005)). Polytetrafluoro-ethylene (PTFE or Gore-Tex®) conduits are currently the most widely used vascular grafts for EC TCPC (Petrossian, et al., J. Thorac. Cardiovasc. Surg., 132:1054-63 (2006)). Use of other synthetic conduits or even biological vascular grafts is described in the literature but to a much more limited extent compared to PTFE (Petrossian, et al., J. Thorac. Cardiovasc. Surg., 117:688-96 (1999)).
While data describing the long-term graft failure rates for conduits used for EC TCPC is limited, long-term data regarding use of both valved and unvalved conduits for other similar congenital heart operations are widely available and are poor (Dearani, et al., Ann. Thorac. Surg., 75:399-411 (2003)). Late problems include conduit degeneration with progressive obstruction, lack of growth potential, increased susceptibility to infection and increased risk for thrombo-embolic complications. Both synthetic and biological conduits are used for these operations. PTFE and other synthetic conduits such as Dacron lack growth potential, necessitating re-operation when a patient outgrows the vascular graft. Synthetic conduits are a significant cause of thrombo-embolic complication due to the large area of synthetic material in contact with blood, which causes activation of the coagulation cascade (Petrossian, et al., J. Thorac. Cardiovasc. Surg., 117:688-96 (1999)). Other clinically available conduits including biological grafts such as homografts and heterografts are associated with significantly lower thromboembolic complication rates compared to synthetic grafts, however; they too lack growth potential and unfortunately have poor durability due to their propensity for accelerated calcific degradation and secondary graft failure (Stark, Peadiatr. Cardiol., 19:282-8 (1998); Cleveland, et al., Circulation, 86(suppl II):II150-3 (1992); Jonas, et al., Circulation, 72(suppl II):II77-83 (1985)). These grafts tend to become stenotic and calcify. This process seems to be immune mediated and more aggressive in younger patients (Karamlou, et al., Eur. J. Cardiothorac. Surg., 27:548-53 (2005)). It is basically assumed that all such conduits will eventually need to be replaced (Bermudez, et al., Ann. Thorac. Surg., 77:881-8 (2004)). Re-operations are associated with significant morbidity and mortality with early post-operative mortality rates around 5% in the best centers (Dearani, et al., Ann. Thorac. Surg., 75:399-411 (2003)). Early and midterm results for these grafts are variable with 5 year patency rates between 65-90%. Long-term data demonstrating graft failure rates between 70-100% at 10-15 years have been reported (Jonas, et al., Circulation, 72(suppl II):II77-83 (1985); Peadiatr. Cardiol., 19:282-8 (1998); Homann, et al., Eur. J. Cardiothorac. Surg., 17:624-30 (2000)). Primary determinants of graft failure include size (with an increased rate of failure in grafts less than 18 mm with another significant drop off below 15 mm) and re-operation (with primary grafts performing better than replacement grafts) (Homann, et al., Eur. J. Cardiothorac. Surg., 17:624-30 (2000)). The best long-term results have been obtained when autologous tissue has been used for or incorporated into the conduit with long-term patency rates exceeding 80% (Bermudez, et al., Ann. Thorac. Surg., 77:881-8 (2004)).
Autografts, conduits created from an individual's own (autologous) tissue, have better long-term effectiveness than any synthetic or biological conduit currently available for use in pediatric cardiovascular surgical applications (Dearani, et al., Ann. Thorac. Surg., 75:399-411 (2003); Bermudez, et al., Ann. Thorac. Surg., 77:881-8 (2004)). Unfortunately autografts are limited in supply, necessitating the use of synthetic or biological conduits in most cases (Homann, et al., Eur. J. Cardiothorac. Surg., 17:624-30 (2000)). Use of synthetic or biological vascular grafts result in increased graft failure rates and increased morbidity and mortality rates when compared to similar operations performed using autologous tissue (Jonas, et al., Circulation, 72(suppl II):II77-83 (1985); Bermudez, et al., Ann. Thorac. Surg., 77:881-8 (2004)).
Complications arising from the use of currently available vascular grafts are a leading cause of postoperative morbidity and mortality after congenital heart surgery (Jonas, et al., J. Thorac. Cardiovasc. Surg., 117:688-96 (1999)). Additionally the lack of growth potential of all currently available vascular conduits is problematic (Alexi-Meskishvili, et al., Eur. J. Cardiothorac. Surg., 18:690-5 (2000)). Use of over-sized grafts in an attempt to avoid outgrowing a conduit is widely practiced. Postponing surgery until the patient is between 2 and 4 years of age, when the diameter of the IVC approaches 60-80% of the adult frequently enables placement of near adult sized conduits (20-22 mm) and limits the need for conduit replacement based on somatic growth alone, however; graft over-sizing is associated with an increased risk of complications (Alexi-Meskishvili, et al., Eur. J. Cardiothorac. Surg., 18:690-5 (2000)). Delaying surgery to minimize the number of re-operations can lead to cardiac dysfunction or even heart failure due to prolonged exposure to volume overload and chronic hypoxia (Petrossian, et al., J. Thorac. Cardiovasc. Surg., 117:688-96 (1999)). Additionally, recent studies have demonstrated marked improvement in somatic growth in patients who undergo surgery at an earlier age, providing further support for the performance of EC TCPC at an earlier age (Ovroutski, et al., Eur. J. Cardiothorac. Surg., 26:1073-9 (2004)). The upper limit of over-sizing is approximately 1.5 times the size of the native vessel after which point over-sizing will cause substantial negative hemodynamic consequences (Lardo, et al., J. Thorac. Cardiovasc. Surg., 117:697-704 (1999)). Recent studies recommend limiting over-sizing conduits to 1.2 times the size of the native vessel because it is thought that the increased risk of thrombo-embolic complications associated with the use of over-sized grafts is greater than the risk of conduit replacement (Alexi-Meskishvili, et al., Eur. J. Cardiothorac. Surg., 18:690-5 (2000)). The development of a vascular graft with growth potential would eliminate this problem and have dramatic implications for the field of congenital heart surgery.
Tissue engineering offers a strategy for constructing autologous grafts and thereby increasing the pool of potential autografts. Using the classical tissue engineering paradigm, autologous cells can be seeded onto a biodegradable tubular scaffold. The scaffold provides sites for cell attachment and space for neotissue formation (Langer and Vacanti, Science, 260:920-6 (1993)). The resulting neotissue can be used for reconstructive surgical applications such as creation of a vascular graft for use in pediatric cardiothoracic operations (Shinoka, et al., J. Thorac. Cardiovasc. Surg., 115:536-46 (1998)). Extensive large animal studies using both ovine and canine animal models, have demonstrated the feasibility of using tissue engineering methodology to construct conduits for use as large caliber grafts in the venous or pulmonary circulation (Shinoka, et al., J. Thorac. Cardiovasc. Surg., 115:536-46 (1998); Watanabe, et al., Tissue Eng., 7(4):429-39 (2001); Matsumura, et al., Biomaterials, 24:2303-8 (2003); Matsumura, et al., Tissue Eng., 12:1-9 (2006)).
Many studies using biodegradable, synthetic scaffolds have employed vascular cells that were isolated from autologous vessel biopsies. More recent studies have explored the use of autologous cells obtained from bone marrow aspirate Matsumura, et al., Biomaterials, 24:2303-8 (2003)). Based in part on the success of animal studies and on the great promise associated with the development of a vascular graft with growth potential for congenital heart surgery, a pilot clinical study was conducted to evaluate the feasibility and safety of using tissue engineered vascular grafts as conduits for EC TCPC in patients with single ventricle cardiac anomalies (Shinoka, et al., New Engl. J. Med., 344(7):532-3 (2001)); Naito, et al., J. Thorac. Cardiovasc. Surg., 125:419-20 (2003)). To date 25 TEVG have been implanted as conduits for EC TCPC with follow-up out through seven years (Shinoka, et al., J. Thorac. Cardiovasc. Surg., 129:1300-8 (2005)). The tissue engineered vascular grafts functioned well without evidence of graft failure. No graft has had to be replaced. There has been no graft related mortality. There have been two graft related complications, which include the development of significant stenosis in two small diameter (<18 mm) conduits. Both were successfully treated, the first with angioplasty and the second with angioplasty and stenting. There were no reported thromboembolic or hemorrhagic complications, infectious complication or evidence of aneurysm formation. Additionally serial imaging demonstrated the growth potential of these grafts. These data support the overall feasibility and safety of this technology.
The methodology of seeding synthetic vascular grafts with autologous cells, however, is still problematic for many reasons. First, it requires an invasive procedure (biopsy) in addition to the need for a substantial period of time in order to expand the cells in culture that limited its clinical utility. This approach also faces the inherent difficulty in obtaining healthy autologous cells from diseased donors (Poh, et al., Lancet, 365:2122-24 (2005); Solan, et al., Cell Transplant., 14(7):481-8 (2005)). The use of cell culture also results in an increased risk of contamination and even the potential for dedifferentiation of the cultured cells. The use of autologous cells to seed the polymeric grafts also limits the off-the-shelf availability of tissue engineered vascular grafts, thereby limiting their overall clinical utility.
Therefore, it is an object of the invention to provide methods for increasing the patency of biodegradable, synthetic vascular grafts without using cell seeding.
It is another object of the invention to provide biodegradable, synthetic vascular grafts with growth potential that have increased patency.